In most modern communications protocols (e.g., in telecommunications), data is sent one bit at one time, sequentially, over a communications channel or computer bus as serial communications. In serial communications, jitter is an unwanted variation of one or more signal characteristics. Jitter may be seen in characteristics such as the interval between successive pulses, or the amplitude, frequency, or phase of successive cycles. Jitter is a significant factor in the design of almost all communications links (e.g., USB, PCI-e, SAS/SATA, XAUI, Infiniband, SONET/SDH, FC, etc.). Jitter is found in a number of signal characteristics (e.g., amplitude, phase, pulse width, or pulse position), and can be quantified in the same terms as all time-varying signals (e.g., root mean square (RMS), peak-to-peak displacement, etc.).
Total jitter (TJ) is generally a specification of serial link standards and is used as a figure of merit when comparing serializer and deserializer (SerDes) devices. TJ comprises data dependent jitter (DDJ). In turn, DDJ comprises inter-symbol interference (ISI) and duty cycle distortion (DCD). Both ISI and DCD affect the amplitude and the phase (e.g., the location of the pulse edge) of a data signal. Left uncompensated, DDJ will cause errors in the received data.
ISI is a form of distortion of a signal that causes both the previously transmitted symbols and the succeeding symbols to have an effect on the currently received symbol. In a digital transmission system, such distortion of the received signal is manifested in the temporal spreading and consequent overlap of individual pulses to the degree that the receiver cannot reliably distinguish between changes of state (e.g., between individual signal elements). ISI is usually an undesirable phenomenon as the surrounding symbols have a similar effect to noise, thus making the communication less reliable. ISI will compromise the integrity of the received data. ISI is usually introduced by bandwidth limited physical media to which a SerDes device is connected and is caused by echoes or non-linear frequency response of the channel.
ISI may be measured by data eye patterns. That is, the relative magnitude of ISI may be viewed by examining the data eye. In this context, the data eye is a pattern generated on an oscilloscope which is triggered by the data clock and displays the received data waveform. The resulting overlay of multiple traces on the oscilloscope produces a shape resembling a partially closed eye. The eye appears more closed when the data is distorted. Methods of combating inter-symbol interference include transmitter side pre-emphasis and receiver side linear equalization and/or DFE. Each of these methods can be either programmed or made adaptive. Thus, the data eye will appear more open when transmission is improved.
DCD is a deviation in duty-cycle value from the ideal (e.g., intended) value or the difference in propagation delay between low to high and high to low delay times. This deviation is both the variance in timing away from an ideal duty cycle (e.g., 50%) and also the variance in average voltage offset. DCD is generally generated within a SerDes device due to an imbalance in drive circuit bias levels. In many serial data systems DCD equates to a deviation in bit time between a 1 bit and a 0 bit. The source of DCD is commonly timing differences between rising and falling edges within a system, but may also be caused by ground shifts in single ended systems.
DDJ also comprises jitter introduced into a serial link via active components. In a fiber optic network, active components include optical amplifiers, couplers, repeaters, etc. These components can actively shape the transmitted waveform, thus introducing additional DDJ.
Both ISI and DCD jitter have significant impact on the performance of a SerDes device since they both serve to reduce the vertical and horizontal openings of a data eye. The vertical dimension of the data eye is referred to as the signal amplitude, measured in millivolts. The horizontal dimension of the data eye is referred to as the signal period and is measured in picoseconds.
Conventional methods of jitter compensation include equalization. At the transmit side, equalization techniques used generally include transmitter pre-emphasis (TXPE) or de-emphasis. TXPE boosts the signal amplitude at the transitions, thus compensating the channel attenuation at high frequencies. Alternatively, lower frequencies may be attenuated (e.g., de-emphasis). For simplicity, any method of transmitter boosting, attenuation, pre-emphasis, or de-emphasis is referred to herein as pre-emphasis.
To achieve extra headroom, a large supply voltage for the transmitter output buffer is required. This leads to increased power consumption. Further, large signal swings also create voltage and current ripples and work as crosstalk aggressors. This gives rise to increased near end crosstalk (NEXT) and/or far end crosstalk (FEXT). In order to restrict crosstalk impact, the transmitted signal dynamic range is limited. As most receivers have some equalization mechanism built-in, it tends to boost the crosstalk energy while boosting the signal energy at high frequency, thus degrading the received signal integrity.
Additionally, TXPE provides only limited DDJ compensation. Since most of the TXPE is focused on data amplitude compensation, the horizontal timing of data pulses is not directly dealt with. This may be seen in optical systems in which active components reshape the pulse waveform.
Other methods of jitter compensation use clock delay elements to control clock edges and data signal transitions. These clock delay elements are difficult to match and are difficult to control over process, temperature, and voltage.
Accordingly, a more comprehensive approach to jitter management is required.